Electrical circuit



April 6, 1943.

E. D. GOODALE ELECTRICAL CIRCUIT Filed Nov. 25, 1941 3 Sheets-Sheet 1 MOM A f +157 F i lz'zze 4 @Q -35 Z ci RF Z A Zoom U \P'Jk -9 a 6 7 37 47 .57 MonzZr 1 I (the I E firr p.

9 49 6:9 I Zine, E il lge- 51%;, i9 L f/Eededaer A M 4 INVENTOR 7% WW ATTORNEY April 6, 1943. E. D. GOODALE 2,315,734

ELECTRICAL CIRCUIT I J Filed Nov. 25, 1941 3 Sheets-Sheet 2 INVENTOR ATTORNEY Patented Apr. 6, 1943 Ehner Dudley Goodale, Bayside, Long Island,

N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application November 25, 1941, Serial No. 420,376

6 Claims.

This invention relates to electrical circuits and, in particular, to a network suitable for use in measuring electrical quantities where it is desired to connect measuring equipment to various points in a system without disturbing the transmission characteristics of the system so that the monitoring equipment depicts-the actual operating conditions.

In electrical transmission circuits it is often necessary to insert measuring equipment at various portions of the circuit in order to determine proper operation or to analyze the current and voltage relationships existing in difl'erent portions of the circuit. To this end, monitoring equipment is provided which may be connected to divers points of the system so that the stepby-step transmission of signals through transmission lines, filters, amplifiers, or the like, can be readily ascertained. One of the difilculties incurred, in such installations known to the prior art, is that the monitoring equipment ordinarily has associated with it a certain value of input impedance, which, when connected into the transmission circuit, absorbs energy therefrom and, consequently, alters the actual current and voltage relationships of the circuit. This disturbance can be minimized for ordinary audio-frequency circuits, by making the input-impedance of the monitoring equipment high relative to the impedance across which it is connected. To this end, the input impedance usually is made to be from ten to fifteen times the impedance across which it is connected.

In audio-frequency circuits any change in electrical phase introduced by monitoring equipment is unimportant ordinarily, since the ear is unable to detect inch phase changes. On the other hand, in television networks the phase relation is of importance comparable to the ampli tude relation of the .currentsand voltages, that is to say, the faithful reproduction of pictures by television requires that the overall system have a substantially linear phase response with relation of the signalling currents. Since, during a television performance, for example, it is necessary to monitor not only the output of the camera amplifiers, but the line amplifiers and the transmission lines, the changing of connections of the monitor to the various points in the television transmission system would cause appropriate changes of the phase relation. At the receiver, there results each time a switch is madewith respect to the monitoring equipment, a sudden change in the reproduced picture, which is'not only distracting but deleterious to, the permonitoring equipment, when the monitoring equipment is connected across the shunt arm.

Moreover, in order that the introduced network shall not change the phase relation, it is necessary that the eilective shunt arm inductance shall be negative so that the network introduces a phase angle which is linear with respect to frequency. The negative inductance is physically unrealizable, but I provide an effective negative inductance by utilizing the mutual inductance between the two series inductances of the series arm of the bridged-T network. By suitably relating the bridging capacity, the series inductance, and the shunt capacity, to each:

other, as well as the transmission line impedance and frequency range, I make it possible to pro vide a network whose insertion introduces subrespect to frequency, as well as a substantially uniform amplitude characteristic with respect to frequency. Thus, while it would be possible to design monitoring equipment whichwould not disturb the amplitude relationships appreciably, difilculties 'do a'rise from the fact. thatfthe inse'rted monitoring'equipmen't chan'gesthe phase stantially 'no insertion loss and which network will possess a linear phase characteristic throughout a given range of frequencies and 01' 'these'networks inserted at thevarious points 2 where measurements by the monitoring equipment are to be made, the connection oi the monitoring equipment across the shunt condenser or capacity will not disturb the actual current and voltage relationships existing in the system, but will provide a measurement of these relationships. Accordingly, it is an important object of my invention to provide a transmission network which has substantially no attenuation and a linear phase characteristic over a relatively wide frequency band between its input and output terminals. I

Another object of my invention is to provide a transmission network to be inserted in a transmission system which does not alter the transmission characteristics with respect to amplitude and in which the effective phase relationship, as a function of frequency, is not disturbed.

A further object of my invention is to provide a transmission network suitable for use in a television system at monitoring positions, such that the connection or disconnection of the minitoring equipment will not disturb the current and voltage relationships existing in the television system.

Still another object of my invention is to provide a new and improved monitoring system for use in television.

Other objects of my invention will become apparent to those skilled in the art upon reading the following detailed description, in which description reference will be made to the drawings.

In the drawings I have shown in Figure 1 a schematic diagram of an illustrative television system with a monitor and the points at which monitoring is usually provided.

Figure 2 shows diagrammatically a lattice network, while Figure 3 shows the equivalent bridged-T network for purposes of describing my invention.

Figure 4 shows schematically thenetwork embodying the principles of my invention together with the monitor connected across the condenser of the shunt arm, while Figure 5 shows in more detail, for purposes of illustration, 9. complete monitoring control board.

In Figure 1 there is shown the monitor I embodying an oscilloscope 3 for observing the signals of a television system. Ordinarily, the monitor I would be maintained in the control room at which point an operator is stationed to insure that the transmissions leave the point of origin in proper fashion.

The various equipment of a television system would comprise, for example, a monoscope 1, a film scanner II, a battery of studio cameras I5, I9, 23, and a radio receiver 21. The monoscope 1 is used to generate a complete television signal, as is well known in the art. The film studio may comprise at least one camera and may include two or more such cameras for scanning motion picture film to generate television signals. The studio cameras ordinarily occur in groups of three, being equipped with long, medium, and

short focal length lenses so as to provide different fields of view of the same scene. The radio receiver is used to pick up programs transmitted from remote points by mobile equipment or transportable equipment. Suitable line amplifiers 3, I3, I1,'2I, 25, and 23 are provided to amplify the signals prior to placing them on the transmission line 61 which runs to the radio transmitter for telecasting.

It is desirable to be able to check the generated signals at the output of the monoscope, film scanner, studio,

and receiver, as well as to monitor the output of the amplifiers associated with this apparatus in order to ascertain whether the transmission from the camera, for example, is proper or 5 faulty, as well as to ascertain that the line amplifieritself is satisfactory. It is also desirable to insure that the signal over the transmission line 61 is proper. Moreover, since ordinarily only 'one signal at a time is passed over the transmission iine, a plurality of switches 5|, 53, 55, 51, 59 and 53 makes it possible to selectively connect at a time but one piece of signal generating equipment to the transmission line. It is also desirable to be able to monitor the equipment even when it is not connected to the transmission line so that in the event that one camera is to be switched to the transmission line while another one is disconnected therefrom that the operator can ascertain beforehand that the equipment to be connected is properly operating. I have shown schematically the usual points of connection for the monitoring equipment in Figure 1 as the points 3|, 33, 35,31, 39, ll, 43, d5, 41, 49, 6|, 65, 69. In order that the connection of the monitoring equipment at these points shall not disturb the equipment and phase characteristic of the system, I place at these points a constant impedance network possessing substantially no attenuation and linear phase 39 characteristic throughout a predetermined wide band of frequencies. Such a network and scheme of connections are shown schematically in Figure 4.

In Figure 4 I have shown a bridged-T network comprising the condenser I2I, the inductances I23, I25, and condenser I21. The two inductances I23, I25, are so wound and coupled together as to provide negative mutual inductance, that is to say, the coils are wound in the same direction and connected in series so that the flux of each coil aids each other. The monitor I3I is then connected across the condenser I21. Provision is made by means of the switch I33 so that when the monitor I3I is connected across the condenser I21-that an auxiliary capacity I35 is disconnected from the condenser I21. The auxiliary condenser I35 has a capacity which is equivalent to the input capacity of the monitor I3I. Under these conditions the total 'eflective shunt arm capacity remains unchanged, and, consequently, the connection or disconnection of the monitor does not change the amplitude or phase of the signaling energy 1passing from the equipment to the transmission The bridged-T network shown in Figure 4 is derived from the lattice network shown in Fig- .ure 2. In Figure 2 one-half of the symmetrical lattice network is shown comprising a series arm of an inductance IBI connected in parallel with the condenser I03., The cross-connected arm comprises the serially connected inductance I05 and the condenser I01. If the cross-connected arm has the value oi. inductance and capacity, such as to be the inverse oi! theseries arm, then the impedance will be constant and the attenuation of the lattice network will be zero, as is well known in the art. In order that there be no reflections at the input and output terminals of the network, when inserted in a. transmission line, the square root of the product of the series arm and the crossarm must beequal to the characteristic impedance of the line, that is, the characteristic impedance of the line is equal to the square root of L/C. A lattice network which will meet these conditions is provided if the inductance II" has a value of mL/2, while inductance I05 has a value of L/2m and the capacities I03 and I01 have values of C/Zm and mC/2, respectively, and where If; and

AIR

where R is the line impedance and in is resonant frequency of L and C. Moreover, it can be shown that the inserted phase. angle 9 of the network, as a function of frequency, is'given by the expression tan 0= function of frequency, the value of m required can be ascertained by taking the first derivative of the phase angle with respect to k and then by Bodes method which is well known in the art, the value of m can be found such that the quotient representing the derivative has a denominator which is equal to or larger than the numerator. The value of m so found will then insure a linear phase shift with respect to frequency over the predetermined. range of frequencies. the value of m found is 1.73. By allowing for a slight departure from linearity the band width can be enlarged appreciably without introducing phase shift difficulties. The lower limit for m under those conditions is 1.60 and ordinarily the preferred is 1.65. Using such a value of m will insure a phase shift which has less than one degree deviation from strict linearity up to at least -36 of the resonant frequency. i. e., w/wo equal to Consequently, the network shown in Figure 2, having the values above described, will have the characteristics of constant impedance, zero attenuation. and linear phase shift over the predetermined range of frequencies for which the network has been designed. By methods well known to the prior art, the lattice network may be transformed to a bridged-T network shown in Figure 3. In Figure 3 the series arm of the lattice network is replaced by a parallelly connected condenser I09 and two serially connected inductances III and H3. The cross arms are replaced by a shunt arm comprising an inductance H5 and a condenser II1. The values of the inductances and capacities are given in the following table:

Capacity I09 equals C/im Capacity II1 equals MC Inductances III and H3 equal mL/2 Inductance II5 equals For the particular network shown,

that their fields aid, with thecoupling so adjusted that the mutual inductance is equal to 4m I with the two Inductances each having a value of mL/2 where a suitable value of m equals 1.65. Since and 1 2 /L C by substitution, a resultant network, as shown in Figure 4, may be derived in terms of R and f, in which f equals 1 equals '158R M equals '050R f C equals C equals .the phase angle is to be linear.

It will be noted that in the network of Figure 4 the capacity C2 actually comprises the condensers I21 and I29 connected in parallel, or. alternatively, the condenser I21 with the monitor I3I connected across it. By making the resistive part of the input impedance of the monitor I3I at least ten times that of the condenser I21, substantially no alteration takes place in the network when the switch I21 is thrown from the condenser I29 to the-monitor I3I. It will thus be appreciated that the network shown in Figure 4 satisfies the necessary conditions of providing substantially exact replica of the signaling energy wave form with respect to time without disturbing the conditions existing at equipment connected to input or output terminals of thenetwork.

In practice it is desirable to effect the connection of'the monitor to the various points of the system by use of push buttons rather than plug and jack. Accordingly, I have shown in Figure 5, by way of example, apparatus for providing in a control booth. for example, for enabling the monitor to be connected to divers points in the television system. In Figure 5 there is shown a plurality of substantially identical bridged-T networks comprising series arms having a condenser -I5I, inductances I53, I55, mutually coupled together. condenser I51 and condenser I59. A push button switch I6I is operated by the push button I1I. .Ordinarily; the arm of the switch I13 is not-in contact with the push button HI and therefore the leaf I61 is in contact with the leaf I69. This leaf completes the connection of the condenser I59 to connect it in parallelwith the condenser I51. When the push button is pressed, the contact portion I13 of the leaf I61 moves to the left to break the contact between the leaves I61 and I69 and to make contact between the leaf I63 and leaf I 61. The opening of the circuit between leaves I61 and I89 disconnects the condenser I59, while the-closing of the contacts between the leaves I63 and I61 connects the monitor across the condenser I51. The

monitor, therefore, would be connected across the network B under such conditions. A sliding release bar I81, of the type well known in the art, is provided so that when any of the buttons l, 2, 3, 4, 5, 6 are pushed, it causes the bar to disengage the monitor from all other filters before it connects the monitor to the desired filter. A release button N33 is also provided so that when it is pressed it disengages the monitor entirely. Such an arrangement has the advantage of providing an operator with a simple and effective means for monitoring the complete television system. It will be understood that in Figure 5, while only six filters are shown, this is merely by way of example. Any number of filters necessary fora system may be used in a fashion similar to that shown in Figure 5.

Various alterations and modifications of the present invention may become apparent to those skilled in the art and it is desirable that any and all such modifications and alterations be considered within the purview of the present invention except as limited by the hereinafter appended claims.

Having now described my invention, what I claim is:

characteristic up to a predetermined maximum frequency-connected in said transmission line,

' said transducer comprising a series arm composed of a condenser connected in parallel with two serially connected inductances, said inductances being coupled to produce negative mutual inductance, a shunt arm comprising a condenser connected to the common junction point of said inductance, and means to connect the input circuit of monitoring equipment across said second named condenser. a

4. A monitoring system comprising a transmission line, a transducer having substantially zero attenuation and a substantially linear phase characteristic up to a predetermined maximum frequency connected across said transmission, said transducer comprising a series arm composed of a condenser connected in parallel with two serially connected inductances, said inductances being coupled to produce negative mutual inductance, a shunt arm comprising a condenser connected to the common Junction point of said inductance, and selective meansto reduce the capacity of said second condenser and to con- 1. A' transfer network having substantially I zero attenuation and a substantially linear phase characteristic up to a predetermined maximum frequency comprising a series arm composed of a condenser connected in parallel with two serially connected inductances, said inductances be-. :ing coupled to produce negative mutual inductance, and a shunt arm comprising a condenser connected to the common junction point of said inductance.

2. A transfer network having substantially zero attenuation comprising a series arm composed of a condenser connected in parallel with two serially connected inductances, said inductances being coupled to produce negative mutual inductanca a shunt arm comprising a condenser connected to the common junction point of said inductance, said first named condenser having a value of .029/Rf farads, said second named condenser having a value of 315/12} farads, said serially connected inductances each having a value of .1158R/f henries, and said mutual inductance having a value of .050R/f, where ,f, is a predetermined maximum frequency up to which the phase characteristic of the network is linear and R is a characteristic impedance of a circuit to which the said network is to be connected. I 3. A monitoring system comprising. a transmission line, a transducer having substantially zero attenuation and a substantially linear phase nect monitoring equipment across said secondnamed condenser.

5. A monitoring system comprising a transmission line, a-transducer having substantially zero attenuation and a substantially linear f phase characteristic up to a predetermined maximum frequency connected across said transmission, said transducer comprising a series arm composed of a condenser connected and parallel with two serially connected inductances, said inductances being coupled to produce negative mutual inductance, a shunt arm comprising a condenser connected to the common junction point of said inductance, and means to substitute monitoring equipment having a predetermined input capacitance for a portion of the capacity of said sec- 0nd named condenser.

6. A monitoring system comprising a plurality of transmission lines to be monitored, a. passive bridged-T network having substantially zero attenuation and a substantially linear phase characteristic up to a predetermined maximum irequency inserted in each of said transmission lines, each of said bridged-T networks having a I shunt arm consisting of two condensers connected in parallel, monitor equipment, and seleotive switching means for substituting said monitoring equipment for one of said condensers,

and means for insuring that one and only one network is connected to said monitor.

E. DUDLEY GOODALE. 

